In general, an electric moor is often used as a drive power source of an onboard apparatus of a vehicle, such as a wiper apparatus, a power seat apparatus or a power window apparatus.
When the electric motor is used as the drive power source of such an onboard apparatus, a thrust load is applied to a rotatable shaft of the electric motor in addition to a radial load.
Therefore, it is necessary to provide a structure that can effectively support the thrust load, which is applied to the shaft of the rotor of the motor, in addition to the radial bearing, which mainly supports the radial load applied to the shaft of the rotor of the motor.
In view of the above need, for example, JPH08-196050A and JP2002-155930A teach structures, which use a thrust ball in a thrust bearing to support the thrust load. The thrust ball reduces the slide loss of the shaft by converting it into rolling friction.
JPH08-196050A teaches a bearing device of a small electric motor.
Specifically, as shown in FIG. 9A, the small electric motor includes a rotor 202 and a side plate 205. A rotatable shaft 203 extends through the rotor 202 and has a rotational central axis.
A conical hole 203c is formed in an end surface of the rotatable shaft 203 made of metal on a side where the side plate 205 is located. A bearing ball 210 made of metal is received in the hole 203c. 
The bearing ball 210 is placed to contact the side plate 205 to form a thrust bearing having a pivot bearing structure.
Furthermore, JP2002-155930A teaches the bearing structure of the rotatable shaft of the electric motor.
Specifically, as shown in FIG. 9B, a radial load of an armature shaft 216 of a geared motor is supported by a metal ring 220, and a thrust load of the armature shaft 216 is supported by a thrust ball 232 held in a holding hole 224.
A shaft hole 222 is formed in the metal ring 220 and receives an end portion of the armature shaft 216. Also, the holding hole 224, which is coaxial with the shaft hole 222 and is communicated with the shaft hole 222, is formed in the metal ring 220. The thrust ball 232 is placed in the holding hole 224.
The thrust ball 232 contacts an end surface of the armature shaft 216 received in the shaft hole 222 and is engaged with an inner end surface of a yoke housing 212 through a thrust plate 234.
Therefore, the armature shaft 216 and the thrust ball 232 are rotatable relative to each other to support the thrust load.
As discussed above with reference to JPH08-196050A and JP2002-155930A, when the structure, which uses the radial bearing and the thrust ball, is used, the thrust load can be effectively supported.
However, according to the technique of JPH08-196050A, it is necessary to form the hole 203c, which is configured into the conical shape (having a V-shaped cross section), in the end portion of the shaft 203, and also it is necessary to place the bearing ball 210 in this hole 203c. 
The hole 203c is made through a cutting process, so that the shaft 203 needs to be made of a material that can be processed through the cutting process.
Particularly, the material, which has the high rigidity, is difficult to process in the cutting process, and thereby it is difficult to choose such a material for the shaft 203.
Furthermore, when the diameter of the shaft 203 is small, the cutting process of the shaft 203 becomes difficult.
In addition, when the diameter of the shaft is small, the diameter of the thrust ball needs to be small. Therefore, the thrust surface pressure is increased, and thereby there is a disadvantage in terms of the durability.
Furthermore, according to the technique of JP2002-155930A, the metal ring 220, which is the radial bearing, is made relatively large to form the holding hole 224, which holds the thrust ball 232. Thus, the costs are disadvantageously increased.
Also, the installation direction of the metal ring 220, which is the radial bearing, is determined to be a specific direction. Therefore, the assembling of the motor using an automatic assembling machine becomes complicated.
Furthermore, according to this technique, the ball 232 may be dislodged from the holding hole 224 unless the shaft 216 is installed to the metal ring 220, which is the radial bearing. Therefore, there is the difficulty in the terms of handling of such a structure.
Furthermore, a shaft edge and the thrust ball interfere with each other in an inside of the metal ring, which is the radial bearing, to cause generation of noises.